Refactor DarkMode management for Controls as per API Review outcome

[KlausLoeffelmann]

For a Control to indicate that it wants to participate in DarkMode theming, it needs to indicate that by setting the style flag ApplyThemingImplicitly.

The original architectural plan was to introduce a dedicate protected virtual method InitializeControl that would be called by the constructor of Control, which would then set the ApplyThemingImplicitly flag and give also the control the chance to learn about the initial DeviceAPI, with that replacing the original internal method.

This approach was not approved by the API Review board (see #13550 (comment)).

Consequently, this PR builds the DarkMode handling back to the original approach and also refactors the flaw of the approach how controls are internally provided the initial DPI setting.
(Also pointed outby the API Review.)

Because of the general architecture of the Controls inheritance hierarchy, Controls need to make sure to opt into (or opt out) DarkMode handling before their own constructor runs, which means they have to override CreateParams and set the respective flags before calling the base implementation.

Microsoft Reviewers: Open in CodeFlow

[Copilot]

Pull Request Overview

This PR reverts a previously unapproved architecture for DarkMode theming in controls and refactors DPI handling to use a PropertyStore‐based approach. Key changes include removing early SetStyle calls from constructors in favor of setting them within CreateParams overrides, replacing the removed InitializeConstantsForInitialDpi method with a standardized InitializeControl override, and migrating from internal DPI field usage to the new DeviceDpiInternal/FormerDeviceDpi properties.

Reviewed Changes

Copilot reviewed 36 out of 36 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
TabControl.cs, Splitter.cs, ProgressBar.cs, PictureBox.cs, MonthCalendar.cs, ListBox.cs, Label.cs	Moved the ApplyThemingImplicitly flag from the constructor to the CreateParams getter and removed redundant pragmas where needed.
PropertyGrid.cs, DataGridView.cs, Control.cs	Updated DPI handling code to use the new PropertyStore keys and properties (DeviceDpiInternal and FormerDeviceDpi), and removed the obsolete InitializeConstantsForInitialDpi.
DataGridViewColumn.cs, ComboBox.FlatComboAdapter.cs	Updated DPI retrieval calls to reference the new properties.
RadioButton.cs, CheckBox.cs, ButtonBase.cs, Button.cs	Replaced the old DPI initialization methods with InitializeControl and changed the accessibility or virtuality of OwnerDraw where applicable.
PublicAPI.Unshipped.txt, PublicAPI.Shipped.txt	Updated and removed API entries to align with the refactored theming and DPI handling changes.

Comments suppressed due to low confidence (1)

src/System.Windows.Forms/System/Windows/Forms/Controls/Buttons/ButtonBase.cs:620

• Changing the accessibility and making OwnerDraw virtual may affect how derived controls override this property. Verify that the new visibility and virtual nature align with the intended usage across the control hierarchy.

private protected virtual bool OwnerDraw => FlatStyle != FlatStyle.System;

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.40072%. Comparing base (f3bc39d) to head (4ed52ca).
Report is 13 commits behind head on main.

Additional details and impacted files

@@                 Coverage Diff                  @@
##                main      #13582          +/-   ##
====================================================
+ Coverage   76.60070%   97.40072%   +20.80002%     
====================================================
  Files           3236        1182        -2054     
  Lines         639344      352713      -286631     
  Branches       47314        5368       -41946     
====================================================
- Hits          489742      343545      -146197     
+ Misses        146026        8412      -137614     
+ Partials        3576         756        -2820     

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integration	?
production	?
test	97.40072% <ø> (+0.00004%)	⬆️
unit	?

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[JeremyKuhne]

Nothing blocking, but the comments should be followed up on.

[JeremyKuhne]

nit: don't leave a blank line between getter and setter blocks, also these should be get =>

[JeremyKuhne]

nit: avoid exclamation points

[JeremyKuhne]

When we always create a value, putting it in the Properties has unnecessary overhead and as such the general recommendation is to not do this for things that aren't optional or are in inner loops.

[KlausLoeffelmann]

I will merge this, so it will not get stale, and will follow up in the next PR.